Typically, solid oxide fuel cells (SOFC) employ an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is sandwiched between a pair of separators (bipolar plates). In use, generally, predetermined numbers of the separators and the electrolyte electrode assemblies are stacked together to form a fuel cell stack.
In the fuel cell, for example, a flat plate type solid oxide fuel cell is adopted. In the flat plate type solid oxide fuel cell, for example, a plate-shaped separator is stacked on an anode of a plate-shaped electrolyte electrode assembly with an anode current collector interposed between the separator and the anode, and a plate-shaped separator is stacked on a cathode thereof with a cathode current collector interposed between the separator and the cathode. In the structure, sponge-like porous material of Ni based alloy or the like is used for the anode current collector. Likewise, sponge-like porous material of Ag based alloy or the like is used for the cathode current collector.
In a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-358980, as shown in FIG. 17, a fuel electrode layer 2a and an air electrode layer 3a are provided respectively on both surfaces of a solid electrolyte layer 1a. A fuel electrode current collector 4a and an air electrode current collector 5a made of porous cushion material are provided respectively on the outer sides of the fuel electrode layer 2a and the air electrode layer 3a. Separators 6a are provided on the outer sides of the fuel electrode current collector 4a and the air electrode current collector 5a, respectively.
Each of the fuel electrode current collector 4a and the air electrode current collector 5a is made of a porous body having gradient composition. More specifically, the composition thereof changes continuously from dense to coarse in the thickness direction. Further, the fuel electrode current collector 4a and the air electrode current collector 5a are interposed respectively between the fuel electrode layer 2a and the separator 6a and between the air electrode layer 3a and the separator 6a such that dense layers having smaller pores are provided at the contact surface between the fuel electrode current collector 4a and the fuel electrode layer 2a and at the contact surface between the air electrode current collector 5a and the air electrode layer 3a, while coarse layers having larger pores are provided at the contact surface between the fuel electrode current collector 4a and the separator 6a and at the contact surface between the air electrode current collector 5a and the separator 6a. 
Further, in a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2008-251238, as shown in FIG. 18, a flat plate type power generation cell 4b having an oxygen-containing gas electrode layer 2b and a fuel electrode layer 3b on both surfaces of a solid electrolyte layer 1b is provided. An oxygen-containing gas electrode current collector 5b and a fuel electrode current collector 6b each made of porous cushion material are provided on the outer sides of the power generation cell 4b. 
Separators 7b are provided on the outer sides of the oxygen-containing gas electrode current collector 5b and the fuel electrode current collector 6b. Ejection ports 8b are provided in the separators 7b. The oxygen-containing gas is supplied from one of the ejection ports 8b to the oxygen-containing gas electrode layer 2b through the oxygen-containing gas electrode current collector 5b, while the fuel gas is supplied from the other ejection port 8b to the fuel electrode layer 3b through the fuel electrode current collector 6b. A hole 9b is formed at a position of the oxygen-containing gas electrode layer 2b that faces the oxygen-containing gas ejection port 8b of the separator 7b. 